Cutting tool

ABSTRACT

A cutting tool comprises a substrate and a coating that coats the substrate, the coating including an α-alumina layer provided on the substrate, the α-alumina layer including crystal grains of α-alumina, the α-alumina layer including a lower portion and an upper portion, the upper portion being occupied in area at a ratio of 50% or more by crystal grains of α-alumina having a (006) plane with a normal thereto having a direction within ±15° with respect to a direction of the normal to the second interface, the lower portion being occupied in area at a ratio of 50% or more by crystal grains of α-alumina having a (110) plane with a normal thereto having a direction within ±15° with respect to the direction of the normal to the second interface, the α-alumina layer having a thickness of 3 μm or more and 20 μm or less.

TECHNICAL FIELD

The present disclosure relates to a cutting tool. The presentapplication claims priority based on Japanese Patent Application No.2019-027257 filed on Feb. 19, 2019. The disclosure in the Japanesepatent application is entirely incorporated herein by reference.

BACKGROUND ART

Cutting tools having a coated substrate have conventionally been used.For example, Japanese Patent Application Laid-Open No. 2004-284003(PTL 1) discloses a surface-coated cutting tool having a coatingincluding an α-Al₂O₃ layer having a total area of 70% or more of crystalgrains presenting a crystal orientation of a (0001) plane in plan viewas observed along a normal to a surface of a layer.

Further, Japanese Patent Laid-Open No. 06-316758 (PTL 2) discloses abody at least partially coated with one or more refractory layers ofwhich at least one layer is alumina, said alumina layer having athickness d=0.5-25 μm and composed of a single-phase α-structure with agrain size (S): 0.5 μm<S<1 μm for 0.5 μm<d<2.5 μm and 0.5 μm<S<3 μm for2.5 μm<d<25 μm.

CITATION LIST Patent Literature

PTL 1: Japanese Patent Laid-Open No. 2004-284003

PTL 2: Japanese Patent Laid-Open No. 06-316758

SUMMARY OF INVENTION

The presently disclosed cutting tool comprises

a substrate and a coating that coats the substrate,

the coating including an α-alumina layer provided on the substrate,

the α-alumina layer including crystal grains of α-alumina,

the α-alumina layer including a lower portion and an upper portion,

the lower portion being a region sandwiched between an imaginary plane Aand an imaginary plane B, the imaginary plane A being an imaginary planewhich passes through a point 0.2 μm away in a direction of thicknessfrom a first interface located on a side of the substrate and isparallel to the first interface, the imaginary plane B being animaginary plane which passes through a point 1.3 μm further away fromthe imaginary plane A in the direction of thickness and is parallel tothe first interface,

the upper portion being a region sandwiched between an imaginary plane Cand an imaginary plane D, the imaginary plane C being an imaginary planewhich passes through a point 0.5 μm away in the direction of thicknessfrom a second interface opposite to the side of the substrate and isparallel to the second interface, the imaginary plane D being animaginary plane which passes through a point 1 μm further away from theimaginary plane C in the direction of thickness and is parallel to thesecond interface,

the first interface being parallel to the second interface,

when a cross section of the α-alumina layer obtained when cut along aplane including a normal to the second interface is subjected to anelectron backscattering diffraction image analysis using a fieldemission scanning microscope to determine a crystal orientation of eachof the crystal grains of α-alumina and a color map is created basedthereon,

then, in the color map,

the upper portion being occupied in area at a ratio of 50% or more bycrystal grains of α-alumina having a (006) plane with a normal theretohaving a direction within ±15° with respect to a direction of the normalto the second interface,

the lower portion being occupied in area at a ratio of 50% or more bycrystal grains of α-alumina having a (110) plane with a normal theretohaving a direction within ±15° with respect to the direction of thenormal to the second interface.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view for illustrating a substrate of a cuttingtool in one manner.

FIG. 2 is a schematic cross section of a cutting tool according to oneembodiment in one manner.

FIG. 3 is a schematic cross section of a cutting tool according to thepresent embodiment in another manner.

FIG. 4 is a part of a color map created based on a cross section of anα-Al₂O₃ layer when a coating is cut along a plane including a normal toa second interface of the α-Al₂O₃ layer.

FIG. 5 is a schematic cross section showing an example of a chemicalvapor deposition apparatus used for manufacturing a coating.

FIG. 6 is a graph representing a ratio in area of the α-Al₂O₃ layer at alower portion occupied by crystal grains for each crystal plane, thecrystal grains having a prescribed crystal plane with a normal theretohaving a direction within ±15° with respect to the direction of thenormal to the second interface.

DETAILED DESCRIPTION Problems to be Solved by the Present Disclosure

In PTLs 1 and 2, a coating including an α-Al₂O₃ layer, as configured asdescribed above, is expected to improve a cutting tool's mechanicalproperties such as wear resistance and breaking resistance, and peelingresistance, and hence increases its lifetime.

However, faster and more efficient cutting processes in recent yearstend to impose increased loads on cutting tools and reduce theirlifetimes. Accordingly, there is a demand for a cutting tool having acoating with further improved mechanical properties.

The present disclosure has been made in view of the above circumstances,and an object thereof is to provide a cutting tool having enhancedpeeling resistance.

Advantageous Effect of the Present Disclosure

According to the present disclosure, a cutting tool with a coatinghaving enhanced peeling resistance can be provided.

DESCRIPTION OF EMBODIMENTS OF THE PRESENT DISCLOSURE

Initially, embodiments of the present disclosure will be listed andspecifically described.

[1] The presently disclosed cutting tool includes

a substrate and a coating that coats the substrate,

the coating including an α-alumina layer provided on the substrate,

the α-alumina layer including crystal grains of α-alumina,

the α-alumina layer including a lower portion and an upper portion,

the lower portion being a region sandwiched between an imaginary plane Aand an imaginary plane B, the imaginary plane A being an imaginary planewhich passes through a point 0.2 μm away in a direction of thicknessfrom a first interface located on a side of the substrate and isparallel to the first interface, the imaginary plane B being animaginary plane which passes through a point 1.3 μm further away fromthe imaginary plane A in the direction of thickness and is parallel tothe first interface,

the upper portion being a region sandwiched between an imaginary plane Cand an imaginary plane D, the imaginary plane C being an imaginary planewhich passes through a point 0.5 μm away in the direction of thicknessfrom a second interface opposite to the side of the substrate and isparallel to the second interface, the imaginary plane D being animaginary plane which passes through a point 1 μm further away from theimaginary plane C in the direction of thickness and is parallel to thesecond interface,

the first interface being parallel to the second interface,

when a cross section of the α-alumina layer obtained when cut along aplane including a normal to the second interface is subjected to anelectron backscattering diffraction image analysis using a fieldemission scanning microscope to determine a crystal orientation of eachof the crystal grains of α-alumina and a color map is created basedthereon,

then, in the color map,

the upper portion being occupied in area at a ratio of 50% or more bycrystal grains of α-alumina having a (006) plane with a normal theretohaving a direction within ±15° with respect to a direction of the normalto the second interface,

the lower portion being occupied in area at a ratio of 50% or more bycrystal grains of α-alumina having a (110) plane with a normal theretohaving a direction within ±15° with respect to the direction of thenormal to the second interface, the α-alumina layer having a thicknessof 3 μm or more and 20 μm or less.

The above cutting tool thus configured has enhanced peeling resistance.As used herein, “peeling resistance” means resistance against thecoating being peeled and falling off the substrate.

[2] The coating further includes an inner layer provided between thesubstrate and the α-alumina layer, and

the inner layer includes TiCN. By this definition, a cutting toolenhanced in wear resistance in addition to peeling resistance can beprovided.

[3] The coating further includes an intermediate layer provided betweenthe inner layer and the α-alumina layer,

the intermediate layer includes a compound composed of: elementaltitanium; and at least one element selected from the group consisting ofcarbon, nitrogen, boron and oxygen, and

the intermediate layer is different in composition from the inner layer.By this definition, adhesion between the substrate and the coating isenhanced, and peeling resistance is further enhanced.

[4] The coating further includes an outermost layer provided on theα-alumina layer, and

the outermost layer includes a compound composed of: an elementaltitanium; and one element selected from the group consisting of carbon,nitrogen and boron. By this definition, a cutting tool excellent in thata coating is identified, in addition to peeling resistance, can beprovided.

[5] The coating has a thickness of 3 μm or more and 30 μm or less. Bythis definition, a cutting tool excellent in wear resistance in additionto peeling resistance can be provided.

DETAILS OF EMBODIMENTS OF THE PRESENT DISCLOSURE

Hereinafter, an embodiment of the present disclosure (hereinafter alsoreferred to as “the present embodiment”) will be described. It should benoted, however, that the present embodiment is not exclusive. In thepresent specification, an expression in the form of “X to Y” means arange's upper and lower limits (that is, X or more and Y or less), andwhen X is not accompanied by any unit and Y is alone accompanied by aunit, X has the same unit as Y. Further, in the present specification,when a compound is represented by a chemical formula with itsconstituent element composition ratio unspecified, such as “TiC,” thechemical formula shall encompass any conventionally known compositionratio (or elemental ratio). The chemical formula shall include not onlya stoichiometric composition but also a nonstoichiometric composition.For example, the chemical formula of “TiC” includes not only astoichiometric composition of “Ti₁C₁” but also a non-stoichiometriccomposition for example of “TiiC_(0.8).” This also applies to compoundsother than “TiC.”

<<Surface-Coated Cutting Tool>>

The presently disclosed cutting tool is

a cutting tool including a substrate and a coating that coats thesubstrate,

the coating including an α-Al₂O₃ layer (α-alumina layer) provided on thesubstrate,

the α-Al₂O₃ layer including crystal grains of α-Al₂O₃ (α-alumina),

the α-Al₂O₃ layer including a lower portion and an upper portion,

the lower portion being a region sandwiched between an imaginary plane Aand an imaginary plane B, the imaginary plane A being an imaginary planewhich passes through a point 0.2 μm away in a direction of thicknessfrom a first interface located on a side of the substrate and isparallel to the first interface, the imaginary plane B being animaginary plane which passes through a point 1.3 μm further away fromthe imaginary plane A in the direction of thickness and is parallel tothe first interface,

the upper portion being a region sandwiched between an imaginary plane Cand an imaginary plane D, the imaginary plane C being an imaginary planewhich passes through a point 0.5 μm away in the direction of thicknessfrom a second interface opposite to the side of the substrate and isparallel to the second interface, the imaginary plane D being animaginary plane which passes through a point 1 μm further away from theimaginary plane C in the direction of thickness and is parallel to thesecond interface,

the first interface being parallel to the second interface,

when a cross section of the α-Al₂O₃ layer obtained when cut along aplane including a normal to the second interface is subjected to anelectron backscattering diffraction image analysis using a fieldemission scanning microscope to determine a crystal orientation of eachof the crystal grains of α-Al₂O₃ and a color map is created basedthereon,

then, in the color map,

the upper portion being occupied in area at a ratio of 50% or more bycrystal grains of α-Al₂O₃ having a (006) plane with a normal theretohaving a direction within ±15° with respect to a direction of the normalto the second interface,

the lower portion being occupied in area at a ratio of 50% or more bycrystal grains of α-Al₂O₃ having a (110) plane with a normal theretohaving a direction within ±15° with respect to the direction of thenormal to the second interface,

the α-Al₂O₃ layer having a thickness of 3 μm or more and 20 μm or less.

A surface-coated cutting tool (hereinafter also simply referred to as a“cutting tool”) 50 of the present embodiment includes a substrate 10 anda coating 40 that coats substrate 10 (see FIG. 2). In one aspect of thepresent embodiment, the coating may coat a rake face of the substrate ormay coat a portion other than the rake face (e.g., a flank face). Thecutting tool can for example be a drill, an end mill, an indexablecutting insert for a drill, an indexable cutting insert for an end mill,an indexable cutting insert for milling, an indexable cutting insert forturning, a metal saw, a gear cutting tool, a reamer, a tap, or the like.

<Substrate>

The substrate of the present embodiment can be any substrateconventionally known as a substrate of this type. For example, itpreferably includes at least one selected from the group consisting of acemented carbide (for example, a tungsten carbide (WC)-base cementedcarbide, a cemented carbide containing Co other than WC, a cementedcarbide with a carbonitride of Cr, Ti, Ta, Nb, or the like other than WCadded, or the like), a cermet (containing Tic, TiN, TiCN, or the like asa major component), a high-speed steel, ceramics (titanium carbide,silicon carbide, silicon nitride, aluminum nitride, aluminum oxide, andthe like), a cubic boron nitride sintered material (a cBN sinteredmaterial), and a diamond sintered material, and more preferably includesat least one selected from the group consisting of cemented carbide,cermet, and cBN sintered material.

Of these various types of substrates, it is particularly preferable toselect a WC-base cemented carbide or a cBN sintered material. This isbecause these substrates are particularly excellent in balance betweenhardness and strength at high temperature, in particular, and presentexcellent characteristics as a substrate for a cutting tool for theabove-described application.

When using a cemented carbide as a substrate, the effect of the presentembodiment is exhibited even if the cemented carbide has a structureincluding free carbon or an extraordinary phase referred to as η phase.Note that the substrate used in the present embodiment may have itssurface modified. For example, for the cemented carbide, the surface maybe provided with a β-free layer, and for the cermet, the surface may beprovided with a surface hardened layer, and even if the surface ismodified in this way the effect of the present embodiment is exhibited.

FIG. 1 is a perspective view for illustrating the substrate of thecutting tool in one manner. A substrate having such a shape is used as,for example, an indexable cutting insert for turning. Substrate 10 has arake face 1, a flank face 2, and a cutting edge ridge portion 3 whererake face 1 and flank face 2 meet. That is, rake face 1 and flank face 2are faces that are connected with cutting edge ridge portion 3interposed therebetween. Cutting edge ridge portion 3 constitutes a tipof a cutting edge of substrate 10. Such a shape of substrate 10 can alsobe understood as a shape of the cutting tool.

When the cutting tool is an indexable cutting insert, substrate 10 alsoincludes a shape with or without a chip breaker. Cutting edge ridgeportion 3 is shaped to include any of a sharp edge (a ridge formed by arake face and a flank face), a honed edge (a sharp edge processed to berounded), a negative land (with beveling applied), and a combination ofthe honed edge and the negative land.

While the shape of substrate 10 and the name of each part thereof havebeen described with reference to FIG. 1, a shape in the cutting toolaccording to the present embodiment that corresponds to substrate 10 andthe name of each part thereof will be indicated by similar terminology.That is, the cutting tool has a rake face, a flank face, and a cuttingedge ridge portion connecting the rake face and the flank face together.

<Coating>

Coating 40 according to the present embodiment includes an α-Al₂O₃ layer20 provided on substrate 10 (see FIG. 2). The “coating” coats at least apart of the substrate (for example, a rake face coming into contact witha workpiece during cutting) to exhibit a function to improve the cuttingtool's various characteristics such as breaking resistance, wearresistance, peeling resistance and the like. The coating is preferablyapplied not only to a part of the substrate but also to the entiresurface of the substrate. However, even if the substrate is partiallyuncoated with the coating or the coating is partially different inconfiguration, such does not depart from the scope of the presentembodiment.

The coating preferably has a thickness of 3 μm or more and 30 μm orless, more preferably 5 μm or more and 25 μm or less. Note that thethickness of the coating means a total in thickness of any layersconstituting the coating. A “layer constituting the coating” includes anα-Al₂O₃ layer, an intermediate layer, an inner layer, an outermostlayer, and the like described hereinafter. For example, the thickness ofthe coating is measured by measuring any 10 points in a sample in across section parallel to the direction of a normal to a surface of thesubstrate with a scanning transmission electron microscope (STEM), andcalculating an average value of the measured 10 points in thickness. Thesame applies when measuring in thickness the α-Al₂O₃ layer, theintermediate layer, the inner layer, the outermost layer, and the likedescribed hereinafter. The scanning transmission electron microscope isJEM-2100F (trade name) manufactured by JEOL Ltd., for example.

(α-Al₂O₃ Layer)

The α-Al₂O₃ layer of the present embodiment includes crystal grains ofα-Al₂O₃ (i.e., aluminium oxide having an α-type crystal structure)(hereinafter also simply referred to as “crystal grains”). In otherwords, the α-Al₂O₃ layer is a layer including polycrystalline α-Al₂O₃.

That is, the α-Al₂O₃ layer may be provided directly on the substrate ormay be provided on the substrate via another layer such as an innerlayer, an intermediate layer, or the like described hereinafter insofaras such does not impair an effect of the cutting tool according to thepresent embodiment. The α-Al₂O₃ layer may be provided thereon withanother layer such as an outermost layer. The α-Al₂O₃ layer may be anoutermost layer (an outermost surface layer) of the coating.

The α-Al₂O₃ layer has the following feature: That is, the α-Al₂O₃ layerincludes a lower portion and an upper portion,

the lower portion being a region sandwiched between an imaginary plane Aand an imaginary plane B, the imaginary plane A being an imaginary planewhich passes through a point 0.2 μm away in a direction of thicknessfrom a first interface located on a side of the substrate and isparallel to the first interface, the imaginary plane B being animaginary plane which passes through a point 1.3 μm further away fromthe imaginary plane A in the direction of thickness and is parallel tothe first interface,

the upper portion being a region sandwiched between an imaginary plane Cand an imaginary plane D, the imaginary plane C being an imaginary planewhich passes through a point 0.5 μm away in the direction of thicknessfrom a second interface opposite to the side of the substrate and isparallel to the second interface, the imaginary plane D being animaginary plane which passes through a point 1 μm further away from theimaginary plane C in the direction of thickness and is parallel to thesecond interface,

the first interface being parallel to the second interface,

when a cross section of the α-Al₂O₃ layer obtained when cut along aplane including a normal to the second interface is subjected to anelectron backscattering diffraction image analysis using a fieldemission scanning microscope to determine a crystal orientation of eachof the crystal grains of α-Al₂O₃ and a color map is created basedthereon,

then, in the color map,

the upper portion being occupied in area at a ratio of 50% or more bycrystal grains of α-Al₂O₃ having a (006) plane with a normal theretohaving a direction within ±15° with respect to a direction of the normalto the second interface (hereinafter also referred to as (006) orientedcrystal grains),

the lower portion being occupied in area at a ratio of 50% or more bycrystal grains of α-Al₂O₃ having a (110) plane with a normal theretohaving a direction within ±15° with respect to the direction of thenormal to the second interface (hereinafter also referred to as (110)oriented crystal grains).

Reference will now be made to FIG. 4 to specifically describe a methodfor creating the color map. FIG. 4 is a part of a color map createdbased on a cross section of the α-Al₂O₃ layer when the coating is cutalong a plane including a normal to the second interface of the α-Al₂O₃layer. A first interface 20 a of α-Al₂O₃ layer 20 shown in FIG. 4 is aninterface located on the side of substrate 10, and a second interface 20b thereof is an interface located opposite to the side of substrate 10.First interface 20 a is parallel to second interface 20 b. When α-Al₂O₃layer 20 is an outermost layer of the coating, second interface 20 bwill be a surface of α-Al₂O₃ layer 20. First interface 20 a is astraight line that passes a center between a straight line L1 thatpasses through a point on the side of the substrate farthest from thesubstrate in the direction of a normal to a major surface of thesubstrate in the color map and is also parallel to the major surface ofthe substrate and a straight line L2 that passes through a point on theside of the substrate closest to the substrate in the same direction andis also parallel to the major surface of the substrate. Second interface20 b is a straight line that passes a center between a straight line M1that passes through a point on the side opposite to the substratefarthest from the substrate in the direction of the normal to the majorsurface of the substrate in the color map and is also parallel to themajor surface of the substrate and a straight line M2 that passesthrough a point on the side opposite to the substrate closest to thesubstrate in the same direction and is also parallel to the majorsurface of the substrate. Note, however, that an apparently unexpectedpoint is excluded in selecting the “point closest to the substrate” andthe “point farthest from the substrate.”

Initially, the α-Al₂O₃ layer is formed on the substrate based on amethod described hereinafter. The formed α-Al₂O₃ layer is cut so as toobtain a cross section perpendicular to the α-Al₂O₃ layer including thesubstrate and the like. That is, the cutting is done so as to expose acut surface of the α-Al₂O₃ layer cut along a plane including a normal tosecond interface 20 b. After that, the cut surface is polished withwaterproof abrasive paper (including SiC grain abrasive as an abrasive).

Note that the cutting is done for example as follows: wax or the like isused to closely fix α-Al₂O₃ layer 20 at a surface thereof (or a surfaceof the coating when another layer is formed on α-Al₂O₃ layer 20) on asufficiently large holding flat plate and thereafter, a rotary bladecutter is used to cut the layer in a direction perpendicular to the flatplate (i.e., cut the layer such that the rotary blade is as vertical aspossible to the flat plate). While this cutting can be performed at anyportion of α-Al₂O₃ layer 20 insofar as it is performed in such avertical direction, it is done preferably in a vicinity of cutting edgeridge portion 3, more preferably in a vicinity of cutting edge ridgeportion 3 at a location where substrate 10 is relatively flat, as willbe described hereinafter.

Furthermore, the cut surface is polished, as described above, with theabove waterproof abrasive paper (with #400, followed by #800 followed by#1500). The numbers (#) of the waterproof abrasive paper meandifferences in grain size of the abrasive, and a larger number indicatesthat the abrasive has a smaller grain size.

Subsequently, the polished surface is further smoothed by ion millingusing Ar ions. The ion milling was performed under the followingconditions:

Acceleration voltage: 6 kVIrradiation angle: 0° from the direction of a normal to the secondinterface of the α-Al₂O₃ layer (that is, the direction of a straightline parallel to the direction of the thickness of the α-Al₂O₃ layer atthe cut surface) Irradiation time: 6 hours

Subsequently the smoothed cross-section (a mirror surface) is observedwith a field emission type scanning electron microscope (FE-SEM)(product name: “SU6600” manufactured by Hitachi High-Tech Corporation)equipped with an electron back-scattered diffractometer (an EBSD device)to obtain an image, which is subjected to an EBSD analysis. While wherethe smoothed cross section is observed is not particularly limited, itis preferably observed in a vicinity of cutting edge ridge portion 3,more preferably in a vicinity of cutting edge ridge portion 3 at alocation where substrate 10 is relatively flat, in view of arelationship with cutting characteristics. The observation with theFE-SEM is conducted at a magnification of 5000 times.

For the EBSD analysis, data is successively collected by positioning afocused electron beam on each pixel individually. A normal to a samplesurface (the smoothed cross section of the α-Al₂O₃ layer) is inclined by70 degrees with respect to the incident beam, and the analysis isconducted at 15 kV. In order to avoid a charging effect, a pressure of10 Pa is applied. A high current mode is used in conformity with anaperture diameter of 60 μm or 120 μm. Data is collected on the crosssection for 200×300 points corresponding to a surface area (anobservation area) of 20 μm (in the direction of the thickness of theα-Al₂O₃ layer)×30 μm (in a direction parallel to an interface of theα-Al₂O₃ layer) in steps of 0.1 μm/step. In doing so, measurement is donein three or more fields of view.

A result of the EBSD analysis is analyzed using commercially availablesoftware (trade name: “Orientation Imaging Microscopy Ver 6.2,”manufactured by EDAX Inc.) to create the color map. Specifically,initially, the crystal orientation of each crystal grain included in thecross section of α-Al₂O₃ layer 20 is determined. The crystal orientationof each crystal grain determined herein is a crystal orientationobserved when each crystal grain appearing in the cross section ofα-Al₂O₃ layer 20 is observed in a plan view in the direction of a normalto that cross section (i.e., a direction penetrating the plane of thesheet of FIG. 4). Then, based on the obtained crystal orientation ofeach crystal grain, the crystal orientation of each crystal grain in thedirection of the normal to a surface of α-Al₂O₃ layer 20 (that is,second interface 20 b) is determined. A color map is created based onthe determined crystal orientation. To create the color map, a “CrystalDirection MAP” method included in the above software can be used. Thecolor map is created throughout the entire area in the direction of thethickness of α-Al₂O₃ layer 20 as observed in the cut surface. Inaddition, a crystal grain partially outside a field of view formeasurement is also counted as one crystal grain.

In FIG. 4, each area surrounded by a solid line and hatched is a (006)oriented crystal grain, and each area surrounded by a solid line anddotted is a (110) oriented crystal grain. Further, each area surroundedby a solid line and unhatched is a crystal grain which does notcorrespond to either the (006) oriented crystal grain or the (110)oriented crystal grain. That is, in FIG. 4, a crystal grain having the(006) plane with a normal thereto having a direction within ±15° withrespect to the direction of a normal to second interface 20 b of α-Al₂O₃layer 20 is hatched. A crystal grain having the (110) plane with anormal thereto having a direction within ±15° with respect to thedirection of the normal to second interface 20 b of α-Al₂O₃ layer 20 isdotted. Further, any crystal grain other than the above two types ofcrystal grains will be unhatched. Furthermore, in FIG. 4, there is anarea shown in black, which is regarded as an area of a crystal grainhaving its crystal orientation undetermined in the above method.

Furthermore, as shown in FIG. 4, α-Al₂O₃ layer 20 includes a lowerportion 20A and an upper portion 20B.

Lower portion 20A is a region sandwiched between an imaginary plane Aand an imaginary plane B, the imaginary plane A being an imaginary planewhich passes through a point 0.2 μm away in a direction of thicknessfrom first interface 20 a located on a side of the substrate and isparallel to the first interface, the imaginary plane B being animaginary plane which passes through a point 1.3 μm further away fromthe imaginary plane A in the direction of thickness and is parallel tothe first interface. That is, a linear distance (or a minimum distance)between the imaginary plane A and the imaginary plane B is 1.3 μm andthis will be the thickness of lower portion 20A. The imaginary plane Aand the imaginary plane B can be set on the created color map based on adistance from the first interface.

Upper portion 20B is a region sandwiched between an imaginary plane Cand an imaginary plane D, the imaginary plane C being an imaginary planewhich passes through a point 0.5 μm away in the direction of thicknessfrom second interface 20 b opposite to the side of the substrate and isparallel to the second interface, the imaginary plane D being animaginary plane which passes through a point 1 μm further away from theimaginary plane C in the direction of thickness and is parallel to thesecond interface. That is, a linear distance (or a minimum distance)between the imaginary plane C and the imaginary plane D is 1 μm and thiswill be the thickness of upper portion 20B. The imaginary plane C andthe imaginary plane D can be set on the created color map based on adistance from the second interface.

In the color map, upper portion 20B is occupied in area at a ratio of50% or more, preferably 50% or more and 90% or less, more preferably 60%or more and 85% or less by crystal grains of α-Al₂O₃ having a (006)plane with a normal thereto having a direction within ±15° with respectto the direction of a normal to second interface 20 b. Herein, a ratioin area is a ratio in area with reference to the entirety in area ofupper portion 20B in the color map.

In the color map, lower portion 20A is occupied in area at a ratio of50% or more, preferably 50% or more and 90% or less, more preferably 60%or more and 80% or less by crystal grains of α-Al₂O₃ having a (110)plane with a normal thereto having a direction within ±15° with respectto the direction of the normal to second interface 20 b. Herein, a ratioin area is a ratio in area with reference to the entirety in area oflower portion 20A in the color map.

Cutting tool 10 having α-Al₂O₃ layer 20 satisfying the aboverequirements can suppress damage to coating peeled as the cutting toolis seized to a workpiece. That is, cutting tool 10 has improved peelingresistance. This will be described in comparison with conventional art.

An approach conventionally taken to improve an α-Al₂O₃ layer'smechanical properties is by controlling a manner of each crystal in asurface of the α-Al₂O₃ layer to improve the α-Al₂O₃ layer'scharacteristics and hence those of a coating having the α-Al₂O₃ layer.Such a conventional approach is based on the idea that a surface of theα-Al₂O₃ layer is a portion subjected to a large load by cutting, andcontrolling the portion's characteristics controls those of the entiretyof the α-Al₂O₃ layer. As such, no attention has been paid to theconfiguration of the Al₂O₃ layer in the direction of the thicknessthereof. For example, while it has been known that an α-Al₂O₃ layerformed in a conventional manufacturing method has a lower portion havingmany crystal grains mainly exhibiting (214) orientation, no attentionhas been paid to controlling the orientation of the crystal grains inthe lower portion.

However, the present inventors have considered that a breakthroughcannot be achieved only by the conventional approach in order to furtherincrease the lifetime of a cutting tool. Accordingly, the presentinventors have conducted a variety of studies focusing on a manner ofeach crystal in the α-Al₂O₃ layer in the direction of the thicknessthereof, and found that a manner of crystals constituting the α-Al₂O₃layer that are located on the side of the substrate significantlycontributes to the α-Al₂O₃ layer's adhesiveness, that is, peelingresistance.

By a further investigation based on the above findings, the presentinventors have obtained the following findings:

(1) the α-Al₂O₃ layer tends to be per se larger in hardness throughoutas a ratio in area thereof occupied by (006) oriented crystal grainsincreases;(2) In contrast, the α-Al₂O₃ layer tends to be less adhesive to anotherlayer when a ratio in area thereof occupied by (006) oriented crystalgrains is excessively large; and(3) Further, the α-Al₂O₃ layer tends to be more adhesive to the otherlayer as a ratio in area thereof occupied by (110) oriented crystalgrains increases.

A cutting tool 50 according to the present embodiment has been completedbased on the above findings, and includes coating 40 having α-Al₂O₃layer 20 having a crystal orientation varying in the direction of thethickness thereof. Specifically, α-Al₂O₃ layer 20 includes upper portion20B occupied in area by (006) oriented crystal grains at a ratio of 50%or more, and lower portion 20A occupied in area by (110) orientedcrystal grains at a ratio of 50% or more.

Such α-Al₂O₃ layer 20 can have upper portion 20B to significantlyenhance the cutting tool in hardness and thus allow the cutting tool tohave large wear resistance. Furthermore, α-Al₂O₃ layer 20 allows highadhesiveness for a layer which is in contact with lower portion 20A. The(110) plane is a crystal plane close to a close-packed plane, and thepresent inventors consider that such high adhesiveness is exhibitedtherefore. Thus, coating 40 of the present embodiment is excellent inpeeling resistance as well as wear resistance, and cutting tool 50 willhave better mechanical properties than conventional.

(Thickness of α-Al₂O₃ Layer)

In the present embodiment, the α-Al₂O₃ layer has a thickness of 3 to 20μm. The α-Al₂O₃ layer preferably has a thickness of 4 to 20 μm, morepreferably 5 to 15 μm. This allows such an excellent effect as above tobe presented. Herein, the thickness of the α-Al₂O₃ layer means ashortest distance from the first interface to the second interface.

When the α-Al₂O₃ layer has a thickness of less than 3 μm, wearresistance attributed to the presence of the α-Al₂O₃ layer tends to beless improved. When the α-Al₂O₃ layer has a thickness exceeding 20 μm,an interfacial stress attributed to a difference in linear expansivitybetween the α-Al₂O₃ layer and another layer increases, and crystalgrains of α-Al₂O₃ may escape. Accordingly, when the α-Al₂O₃ layer has amiddle portion between the upper portion and the lower portion, themiddle portion will preferably have a thickness of 0 to 17 μm. Such athickness can be confirmed by observing a vertical cross section of thesubstrate and the coating with a scanning transmission electronmicroscope (STEM) or the like, similarly as has been described above.

(Average Grain Size of Crystal Grains of α-Al₂O₃)

In the present embodiment, the crystal grains of α-Al₂O₃ preferably havean average grain size of 0.1 to 3 μm, and more preferably 0.2 to 2 μm.In one aspect of the present embodiment, the lower portion of α-Al₂O₃includes crystal grains having an average grain size preferably of 0.1to 2 μm, more preferably 0.1 to 1 μm. The crystal grain's average grainsize can be determined from the color map for example. Specifically,initially, in the color map, a region which matches in color (that is,in crystal orientation) and is surround by a different color (that is,by a different crystal orientation) is regarded as a crystal grain'sindividual region. Subsequently, the crystal grain's area is determinedand the diameter of a circle having the same area as that of the crystalgrain is defined as the grain size of the crystal grain.

(Inner Layer)

Preferably, coating 40 further includes an inner layer 21 providedbetween substrate 10 and α-Al₂O₃ layer 20 (see FIG. 3), and inner layer21 includes TiCN. The TiCN preferably has a cubic crystal structure.Such an inner layer contains many TiCN crystals of (211) planeorientation. Therefore, it exhibits a strong adhesive force for theα-Al₂O₃ layer including a lower portion occupied in area by (110)oriented crystal grains at a ratio of 50% or more. As a result, thecoating's peeling resistance is further improved.

The inner layer preferably has a thickness of 3 to 20 μm, morepreferably 5 to 15 μm. Such a thickness can be confirmed by observing avertical cross section of the substrate and the coating with a scanningtransmission electron microscope (STEM) or the like, similarly as hasbeen described above.

(Intermediate Layer)

Preferably, coating 40 further includes an intermediate layer 22provided between inner layer 21 and α-Al₂O₃ layer 20 (see FIG. 3) andintermediate layer 22 includes a compound composed of: elementaltitanium; and at least one element selected from the group consisting ofC (carbon), N (nitrogen), B (boron) and O (oxygen). The intermediatelayer is different in composition from the inner layer.

Examples of the compound included in the intermediate layer includeTiCNO, TiBN, and TiB₂.

The intermediate layer preferably has a thickness of 0.3-2.5 μm, morepreferably 0.5-1 μm. Such a thickness can be confirmed by observing avertical cross section of the substrate and the coating with a scanningtransmission electron microscope (STEM) or the like, similarly as hasbeen described above.

(Outermost Layer)

Preferably, coating 40 further includes an outermost layer 23 providedon α-Al₂O₃ layer 20 (see FIG. 3) and outermost layer 23 includes acompound composed of: elemental titanium; and one element selected fromthe group consisting of C, N and B.

Examples of the compound included in the outermost layer include TiC,TiN, and TiB₂.

The outermost layer preferably has a thickness of 0.1-1 μm, morepreferably 0.3-0.8 μm. Such a thickness can be confirmed by observing avertical cross section of the substrate and the coating with a scanningtransmission electron microscope (STEM) or the like, similarly as hasbeen described above.

(Another Layer)

The coating may further include another layer insofar as it does notimpair an effect of the cutting tool according to the presentembodiment. The other layer may have a composition different from oridentical to that of the α-Al₂O₃ layer, the inner layer, theintermediate layer or the outermost layer. Examples of the compoundincluded in the other layer include TiN, TiCN, TiBN, and Al₂O₃ forexample. The other layer is not limited, either, in in what order it isstacked. While the other layer is not particularly limited in thicknessas long as it does not impair an effect of the present embodiment, it isfor example 0.1 μm or more and 20 μm or less.

<<Method for Manufacturing a Surface-Coated Cutting Tool>>

A method for manufacturing a cutting tool according to the presentembodiment includes:

a first step of preparing the substrate (hereinafter also simplyreferred to as a “first step”);

a second step of forming a lower portion of the α-Al₂O₃ layer on thesubstrate through chemical vapor deposition using a source gas includingcarbon dioxide gas and hydrogen sulfide gas (hereinafter also simplyreferred to as a “second step”); and

a third step of forming an upper portion of the α-Al₂O₃ layer on thelower portion through chemical vapor deposition using a source gasincluding carbon dioxide gas and hydrogen sulfide gas (hereinafter alsosimply referred to as a “third step”), wherein 1.5≤(R1/R2)≤2 beingsatisfied, where R1 represents a ratio in volume of the carbon dioxidegas to the hydrogen sulfide gas (CO₂/H₂S) in the second step, and R2represents a ratio in volume of the carbon dioxide gas to the hydrogensulfide gas (CO₂/H₂S) in the third step.

The middle portion can be understood as a “transition portion” formedwhile the second step is shifted to the third step.

<First Step: Step of Preparing a Substrate>

In the first step, a substrate is prepared. For example, a cementedcarbide substrate is prepared as the substrate. The cemented carbidesubstrate may be a commercially available product or may be manufacturedin a typical powder metallurgy method. When manufacturing the substratein a typical powder metallurgy method, for example, WC powder and Copowder are mixed using a ball mill or the like to obtain a powderymixture. After the powdery mixture is dried, it is shaped into aprescribed shape to obtain a shaped body. The shaped body is sintered toobtain a WC—Co based cemented carbide (a sintered material).Subsequently, the sintered material can be honed or subjected to aprescribed process for a cutting edge to prepare a substrate made of theWC—Co based cemented carbide. In the first step, any other substrate maybe prepared insofar as it is a substrate conventionally known as asubstrate of this type.

<Second Step: Step of Forming Lower Portion of α-Al₂O₃ Layer>

In the second step, a lower portion of the α-Al₂O₃ layer is formed onthe substrate through chemical vapor deposition (CVD) using a source gasincluding carbon dioxide gas and hydrogen sulfide gas.

FIG. 5 is a schematic cross section showing an example of a chemicalvapor deposition (CVD) apparatus used for manufacturing a coating.Hereinafter, the second step will be described with reference to FIG. 5.A CVD apparatus 30 includes a plurality of substrate setting jigs 31 forholding substrate 10, and a reaction chamber 32 made of heat-resistantalloy steel and covering substrate setting jigs 31. A temperaturecontroller 33 is provided around reaction chamber 32 for controlling thetemperature inside reaction chamber 32. Reaction chamber 32 is providedwith a gas introduction pipe 35 having a gas introduction port 34. Gasintroduction pipe 35 is disposed in an internal space of reactionchamber 32 in which substrate setting jig 31 is disposed, such that gasintroduction pipe 35 extends in the vertical direction rotatably aboutthe vertical direction, and is also provided with a plurality of jettingholes 36 for jetting gas into reaction chamber 32. CVD apparatus 30 canbe used to form the lower portion and the like of the α-Al₂O₃ layerconstituting the coating, as follows:

Initially, substrate 10 is placed on substrate setting jig 31, and asource gas for the lower portion of α-Al₂O₃ layer 20 is introduced intoreaction chamber 32 through gas introduction pipe 35 while controllingtemperature and pressure inside reaction chamber 32 to each fall withina prescribed range. Thus, lower portion 20A of α-Al₂O₃ layer 20 isformed on substrate 10. Note that, preferably, before forming lowerportion 20A of α-Al₂O₃ layer 20 (that is, before the second step), aninner layer (a layer including TiCN (not shown)) is formed on a surfaceof substrate 10 by introducing a source gas for the inner layer throughgas introduction pipe 35 into reaction chamber 32. Hereinafter will bedescribed a method for forming lower portion 20A of α-Al₂O₃ layer 20after the inner layer is formed on the surface of substrate 10.

While the source gas for the inner layer is not particularly limited, anexample thereof is a gaseous mixture of TiCl₄, N₂, and CH₃CN.

In forming the inner layer, reaction chamber 32 is preferably controlledin temperature to 1000 to 1100° C. In forming the inner layer, reactionchamber 32 is preferably controlled in pressure to 0.1 to 1013 hPa. Notethat H₂ is preferably used as a carrier gas. When introducing gas, it ispreferable to rotate gas introduction pipe 35 by a drive unit (notshown). This allows each gas to be uniformly dispersed in reactionchamber 32.

Further, the inner layer may be formed through MT (MediumTemperature)-CVD. In contrast to CVD performed at a temperature of 1000to 1100° C. (hereinafter also referred to as “HT-CVD”), MT-CVD is amethod to form a layer by maintaining the temperature inside reactionchamber 32 at a relatively low temperature of 850 to 950° C. SinceMT-CVD is performed at a lower temperature than HT-CVD, it can reducedamage to substrate 10 caused by heating. In particular, when the innerlayer is a TiCN layer, it is preferable to form it through MT-CVD.

Subsequently, lower portion 20A of α-Al₂O₃ layer 20 is formed on theinner layer. As the source gas, a gaseous mixture of AlCl₃, HCl, CO,CO₂, and H₂S is used for example.

The source gas preferably contains 0.1 to 6% by volume, more preferably0.5 to 3% by volume, still more preferably 0.6 to 2.5% by volume of CO₂.CO₂ preferably has a flow rate of 0.1 to 4 L/min.

The source gas preferably contains 0.1 to 1% by volume, more preferably0.5 to 1% by volume, still more preferably 0.5 to 0.8% by volume of H₂S.H₂S preferably has a flow rate of 0.1 to 0.5 L/min.

A ratio in volume of CO₂ to H₂S (CO₂/H₂S) is preferably 0.5 to 4, morepreferably 0.5 to 2.

The source gas preferably contains 2 to 5% by volume, more preferably 3to 4% by volume of AlCl₃. AlCl₃ preferably has a flow rate of 2.2 L/minfor example.

The source gas preferably contains 1 to 4% by volume, more preferably 2to 3.5% by volume of HCl. HCl preferably has a flow rate of 2 L/min forexample.

The source gas preferably contains 0.1 to 4% by volume of CO. COpreferably has a flow rate of 0.1 to 2 L/min.

Reaction chamber 32 is preferably controlled in temperature to 950 to1000° C. Reaction chamber 32 is preferably controlled in pressure to 50to 100 hPa. Controlling the temperature within the above rangefacilitates forming a fine grain structure of α-Al₂O₃. H₂ can be used asa carrier gas. It is preferable, similarly as has been described above,to rotate gas introduction pipe 35 when introducing the gases.

In the above manufacturing method, each layer is varied in manner bycontrolling each condition for CVD. For example, each layer'scomposition is determined by a composition of a source gas introducedinto reaction chamber 32. Each layer is controlled in thickness byexecution time (or deposition time). In particular, in order to reduce aratio of coarse grains in α-Al₂O₃ layer 20 or increase (006) planeorientation in the third step described hereinafter, it is important tocontrol a flow rate ratio of CO₂ gas and H₂S gas (CO₂/H₂S) in the sourcegas.

<Third Step: Step of Forming Upper Portion of α-Al₂O₃ Layer>

In the third step, an upper portion of the α-Al₂O₃ layer is formed onthe lower portion through chemical vapor deposition using a source gasincluding carbon dioxide gas and hydrogen sulfide gas.

As the source gas, a gaseous mixture of AlCl₃, HCl, CO₂, and H₂S is usedfor example.

The source gas preferably contains 0.15 to 8% by volume, more preferably0.5 to 3% by volume, still more preferably 0.6 to 2.5% by volume of CO₂.CO₂ preferably has a flow rate of 0.1 to 4 L/min.

The source gas preferably contains 0.15 to 1% by volume, more preferably0.5 to 1% by volume, more preferably 0.5 to 0.8% by volume of H₂S. H₂Spreferably has a flow rate of 0.1 to 0.5 L/min.

A ratio in volume of CO₂ to H₂S (CO₂/H₂S) is preferably 0.5 to 4, morepreferably 0.5 to 2.

Furthermore it is preferable to satisfy 1.5<(R1/R2)<2, where R1represents a ratio in volume of CO₂ to H₂S (CO₂/H₂S) in the second step,and R2 represents a ratio in volume of CO₂ to H₂S (CO₂/H₂S) in the thirdstep.

The source gas preferably contains 6 to 12% by volume, more preferably 7to 10% by volume of AlCl₃. AlCl₃ preferably has a flow rate of 4.5 L/minfor example.

The source gas preferably contains 1 to 4% by volume, more preferably1.5 to 3% by volume of HCl. HCl preferably has a flow rate of 1 L/minfor example.

Reaction chamber 32 is preferably controlled in temperature to 950 to1000° C. Reaction chamber 32 is preferably controlled in pressure to 50to 100 hPa. Controlling the temperature within the above rangefacilitates growth of crystal grains of α-Al₂O₃. H₂ can be used as acarrier gas. It is preferable, similarly as has been described above, torotate gas introduction pipe 35 when introducing the gas.

<Another Step>

In the manufacturing method according to the present embodiment, inaddition to the steps described above, an additional step may beperformed, as appropriate, within a range that does not impair an effectof the present embodiment. Examples of the additional step include thestep of forming an intermediate layer between the inner layer and theα-Al₂O₃ layer, the step of forming an outermost layer on the α-Al₂O₃layer, the step of blasting the coating, and the like. The intermediatelayer and the outermost layer may be formed in any method, and thelayers are formed for example through CVD.

What has been described above includes features given in the followingnotes.

(Note 1)

A surface-coated cutting tool comprising a substrate and a coating thatcoats the substrate,

the coating including an α-Al₂O₃ layer provided on the substrate,

the α-Al₂O₃ layer including crystal grains of α-Al₂O₃,

the α-Al₂O₃ layer including a lower portion and an upper portion,

the lower portion being a region sandwiched between an imaginary plane Aand an imaginary plane B, the imaginary plane A being an imaginary planewhich passes through a point 0.2 μm away in a direction of thicknessfrom a first interface located on a side of the substrate and isparallel to the first interface, the imaginary plane B being animaginary plane which passes through a point 1.3 μm further away fromthe imaginary plane A in the direction of thickness and is parallel tothe first interface,

the upper portion being a region sandwiched between an imaginary plane Cand an imaginary plane D, the imaginary plane C being an imaginary planewhich passes through a point 0.5 μm away in the direction of thicknessfrom a second interface opposite to the side of the substrate and isparallel to the second interface, the imaginary plane D being animaginary plane which passes through a point 1 μm further away from theimaginary plane C in the direction of thickness and is parallel to thesecond interface,

the first interface being parallel to the second interface,

when a cross section of the α-Al₂O₃ layer obtained when cut along aplane including a normal to the second interface is subjected to anelectron backscattering diffraction image analysis using a fieldemission scanning microscope to determine a crystal orientation of eachof the crystal grains of α-Al₂O₃ and a color map is created basedthereon,

then, in the color map,

the upper portion being occupied in area at a ratio of 50% or more bycrystal grains of α-Al₂O₃ having a (006) plane with a normal theretohaving a direction within ±15° with respect to a direction of the normalto the second interface,

the lower portion being occupied in area at a ratio of 50% or more bycrystal grains of α-Al₂O₃ having a (110) plane with a normal theretohaving a direction within ±15° with respect to the direction of thenormal to the second interface.

(Note 2)

The surface-coated cutting tool according to note 1, wherein the α-Al₂O₃layer has a thickness of 3 μm or more and 20 μm or less.

(Note 3)

The surface-coated cutting tool according to note 1 or 2, wherein

the coating further includes an inner layer provided between thesubstrate and the α-Al₂O₃ layer, and

the inner layer includes TiCN.

(Note 4)

The surface-coated cutting tool according to any one of notes 1 to 3,wherein

the coating further includes an intermediate layer provided between theinner layer and the α-Al₂O₃ layer,

the intermediate layer includes a compound composed of: elementaltitanium; and at least one element selected from the group consisting ofC, N, B and O, and

the intermediate layer is different in composition from the inner layer.

(Note 5)

The surface-coated cutting tool according to any one of notes 1 to 4,wherein

the coating further includes an outermost layer provided on the α-Al₂O₃layer, and

the outermost layer includes a compound composed of: an elementaltitanium; and one element selected from the group consisting of C, N andB.

(Note 6)

The surface-coated cutting tool according to any one of notes 1 to 5,wherein the coating has a thickness of 3 μm or more and 30 μm or less.

EXAMPLES

Hereinafter, the present invention will more specifically be describedwith reference to examples although the present invention is not limitedthereto.

<<Manufacturing a Cutting Tool>>

<First Step: Step of Preparing a Substrate>

As a substrate was prepared a cutting insert of cemented carbide (shape:CNMG120408 N-UX, manufactured by Sumitomo Electric Hardmetal Ltd., JISB4120 (2013)) having a composition composed of TaC (2.0% by mass), NbC(1.0% by mass), Co (10.0% by mass) and WC (balance) (includingunavoidable impurity).

<Step of Forming Inner and Intermediate Layers>

Before the second step described hereinafter is performed, a CVDapparatus was employed to form an inner layer and an intermediate layerin the stated order on the prepared substrate. Each layer was formedunder the conditions indicated below. Note that a value in parenthesesfollowing each gas composition indicates a flow rate (L/min) of eachgas. Table 1 shows the thickness of the inner layer and that of theintermediate layer, and the composition of the intermediate layer foreach sample.

(Inner Layer: TiCN)

Source gas: TiCl₄ (10 L/min), CH₃CN (1.5 L/min), N₂ (15 L/min), H₂ (80L/min)

Pressure: 100 hPa Temperature: 850° C.

Deposition Time: adjusted, as appropriate, to provide thickness shown inTable 1.

(Intermediate Layer: TiCNO, TiCN, or TiBN)

(For TiCNO)

Source gas: TiCl₄ (0.4 L/min), CH₄ (2.5 L/min), CO (0.5 L/min), N₂ (25L/min), H₂ (50 L/min)

Pressure: 140 hPa Temperature: 970° C.

Deposition Time: adjusted, as appropriate, to provide thickness shown inTable 1.

(For TiCN)

Source gas: TiCl₄ (4 L/min), CH₃CN (2 L/min), N₂ (30 L/min), H₂ (70L/min)

Pressure: 80 hPa Temperature: 980° C.

Deposition Time: adjusted, as appropriate, to provide thickness shown inTable 1.

(For TiBN)

Source gas: TiCl₄ (5 L/min), BCl₃ (0.5 L/min), N₂ (25 L/min), H₂ (60L/min)

Pressure: 65 hPa Temperature: 970° C.

Deposition Time: adjusted, as appropriate, to provide thickness shown inTable 1.

<Second Step: Step of Forming a Lower Portion of α-Al₂O₃ Layer>

A CVD apparatus was employed to form a lower portion of the α-Al₂O₃layer on the substrate having the inner and intermediate layers formedthereon, followed by a subsequent, third step. The lower portion of theα-Al₂O₃ layer was formed under the following conditions. Table 1 showsthe thickness of the α-Al₂O₃ layer for each example.

(Lower Portion of α-Al₂O₃ Layer)

Source gas: AlCl₃ (2.2 L/min), CO₂ (0.1 to 4.0 L/min), CO (0.1 to 2.0L/min), H₂S (0.1 to 0.5 L/min), HCl (2.0 L/min), H₂ (60 L/min)

Pressure: 50 to 100 hPa Temperature: 950 to 1000° C.

Deposition Time: adjusted, as appropriate, to provide the lower portionwith a thickness of 2 micrometers.

<Third Step: Step of Forming an Upper Portion of α-Al₂O₃ Layer>

Subsequently, a CVD apparatus was employed to form an upper portion ofthe α-Al₂O₃ layer on the substrate that had formed thereon the lowerportion of the α-Al₂O₃ layer to thus form the α-Al₂O₃ layer. The upperportion of the α-Al₂O₃ layer was formed under the following conditions.Table 1 shows a ratio R1/R2, where R1 represents a ratio in volume ofCO₂ to H₂S (CO₂/H₂S) in the second step, and R2 represents a ratio involume of CO₂ to H₂S (CO₂/H₂S) in the third step.

(Upper Portion of α-Al₂O₃ Layer)

Source gas: AlCl₃ (4.5 L/min), CO₂ (0.1 to 4.0 L/min), H₂S (0.1 to 0.5L/min), HCl (1.0

L/min), H₂ (50 L/min) Pressure: 50 to 100 hPa Temperature: 950-1000° C.

Deposition Time: adjusted, as appropriate, so that the lower and upperportions together have a total thickness as shown in Table 1.

TABLE 1 CO₂/H₂S when inner layer providing (TiCN layer) intermediatelayer α-Al₂O₃ layer outermost layer deposition samples thickness (μm)thickness (μm) thickness (μm) thickness (μm) R1/R2 1 7.1 TiCNO (0.5) 6.2TiN (0.8) 1.6 2 8.5 TiBN (0.5) 13 TiB₂ (0.4) 1.7 3 13.2 TiCN (0.8) 8.5TiC (0.4) 1.6 4 8.5 TiBN (0.3) 4.5 TiN (0.6) 1.8 5 5.4 TiCNO (0.5) 19TiC (0.3) 1.7 6 3.2 TiBN (0.8) 20.2 TiN (0.6) 1.9 7 8.2 TiCNO (0.8) 6.5None 1.6 8 4.4 TiCNO (0.5) 7.5 None 1.9 9 5.5 TiCNO (1.2) 6.2 TiC (0.7)2 10 7.1 TiBN (2.5) 9.8 TiB₂ (0.4) 1.8 11 6.2 TiCNO (1.2) 14.2 TiC (0.6)1.6 12 8.9 TiCNO (1.5) 6.8 TiB₂ (0.3) 1.7 a 5.3 TiCNO (0.5) 5.1 TiN(0.5) 2.1 b 6.4 TiBN (0.3) 3 TiB₂ (0.5) 2.4 c 6.1 TiCNO (0.5) 3.2 None 3d 9.5 TiBN (0.3) 4.1 TiN (0.3) 3.2 e 6 TiCNO (0.8) 8.2 TiC (0.3) 2.8 f7.2 TiCNO (0.7) 3.8 TiN (0.2) 2.9

<Step of Forming an Outermost Layer>

Finally, a CVD apparatus was employed to form an outermost layer on thesubstrate having the α-Al₂O₃ layer formed thereon (except for Samples 7,8 and c).

The outermost layer was formed under the conditions indicated below.Table 1 shows the thickness and composition of the outermost layer foreach sample.

(Outermost layer: TiN, TiC, or TiB₂)

(For TiN)

Source gas: TiCl₄ (5 L/min), N₂ (25 L/min), H₂ (70 L/min)

Pressure: 150 hPa Temperature: 980° C.

Deposition Time: adjusted, as appropriate, to provide thickness shown inTable 1.

(For TiC)

Source gas: TiCl₄ (2 L/min), CH₄ (4 L/min), H₂ (80 L/min)

Pressure: 350 hPa Temperature: 990° C.

Deposition Time: adjusted, as appropriate, to provide thickness shown inTable 1.

(For TiB₂)

Source gas: TiCl₄ (15 L/min), BCl₃ (0.2 L/min), H₂ (75 L/min)

Pressure: 400 hPa Temperature: 1000° C.

Deposition Time: adjusted, as appropriate, to provide thickness shown inTable 1.

Through the above procedure, cutting tools for samples 1 to 12 andsamples a to f were manufactured.

<<Evaluating Characteristics of Cutting Tools>>

Using the cutting tools of the samples manufactured as described above,the cutting tools' characteristics were evaluated as follows: Note thatsamples 1 to 12 correspond to examples, and samples a to f correspond tocomparative examples.

<Creating a Color Map>

Initially, the cutting tool was cut so that a cross sectionperpendicular to a surface (or an interface) of the α-Al₂O₃ layer in thecoating was obtained. Subsequently, the cut surface was polished withwaterproof abrasive paper (manufactured by Noritake Coated Abrasive Co.,Ltd. (NCA), trade name: WATERPROOF PAPER, #400, #800, #1500) to preparea processed surface of the α-Al₂O₃ layer. Subsequently, the processedsurface is further smoothed by ion milling using Ar ions. The ionmilling was performed under the following conditions: Accelerationvoltage: 6 kV

Irradiation angle: 0° from the direction of a normal to the secondinterface of the α-Al₂O₃ layer (that is, the direction of a straightline parallel to the direction of the thickness of the α-Al₂O₃ layer inthe cut surface)Irradiation time: 6 hours

The thus prepared processed surface was observed with an EBSD equippedFE-SEM (trade name: “SU6600” manufactured by Hitachi High-TechnologiesCorporation) at a magnification of 5000 times to create a color map, asdescribed above, of the processed surface for an observation area of 20μm (in the direction of the thickness of the α-Al₂O₃ layer)×30 μm (in adirection parallel to an interface of the α-Al₂O₃ layer). Three suchcolor maps were created (in other words, measurement was done in threefields of view). Specifically, initially, the crystal orientation ofeach crystal grain included in the cross section of the α-Al₂O₃ layerwas determined. The crystal orientation of each crystal grain asdetermined herein is a crystal orientation observed when each crystalgrain appearing in the cross section of the α-Al₂O₃ layer is viewed in aplan view in the direction of a normal to that cross section (that is, adirection penetrating the plane of the sheet of FIG. 4). Based on thecrystal orientation of each crystal grain determined, the crystalorientation of each crystal grain in the direction of the normal to thesecond interface of the α-Al₂O₃ layer was determined. A color map wascreated based on the determined crystal orientation (for example, seeFIG. 4). For each color map, a ratio in area of the α-Al₂O₃ layer at thelower portion occupied by (110) oriented crystal grains, a ratio in areaof the α-Al₂O₃ layer at the lower portion occupied by (214) orientedcrystal grains, and a ratio in area of the α-Al₂O₃ layer at the upperportion occupied by (006) oriented crystal grains were determined usingcommercially available software (trade name: “Orientation ImagingMicroscopy Ver 6.2” manufactured by EDAX Inc.). A result thereof isshown in table 2 and FIG. 6. Note that the lower portion is a regionsandwiched between an imaginary plane A and an imaginary plane B, theimaginary plane A being an imaginary plane which passes through a point0.2 μm away in a direction of thickness from a first interface locatedon a side of the substrate and is parallel to the first interface, theimaginary plane B being an imaginary plane which passes through a point1.3 μm further away from the imaginary plane A in the direction ofthickness and is parallel to the first interface (see FIG. 4 forexample). The upper portion is a region sandwiched between an imaginaryplane C and an imaginary plane D, the imaginary plane C being animaginary plane which passes through a point 0.5 μm away in thedirection of thickness from a second interface opposite to the side ofthe substrate and is parallel to the second interface, the imaginaryplane D being an imaginary plane which passes through a point 1 μmfurther away from the imaginary plane C in the direction of thicknessand is parallel to the second interface (see FIG. 4 for example).

Herein, the first interface and the second interface were defined in thecolor map, as follows: Initially, in the color map, the area of theα-Al₂O₃ layer and that other than the α-Al₂O₃ layer were differentlycolored and displayed so that they were distinguishable. A straight linethat passes a center between a straight line L1 that passes through apoint on the side of the substrate farthest from the substrate in thedirection of a normal to a major surface of the substrate in the colormap and is also parallel to the major surface of the substrate and astraight line L2 that passes through a point on the side of thesubstrate closest to the substrate in the same direction and is alsoparallel to the major surface of the substrate is defined as the firstinterface. A straight line that passes a center between a straight lineM1 that passes through a point on the side opposite to the substratefarthest from the substrate in the direction of the normal to the majorsurface of the substrate in the color map and is also parallel to themajor surface of the substrate and a straight line M2 that passesthrough a point on the side opposite to the substrate closest to thesubstrate in the same direction and is also parallel to the majorsurface of the substrate is defined as the second interface.

FIG. 6 is a graph representing a ratio in area of the α-Al₂O₃ layer ofsample no. 8 at a lower portion occupied by crystal grains for eachcrystal plane, the crystal grains having a prescribed crystal plane witha normal thereto having a direction within ±15° with respect to thedirection of the normal to the second interface. Crystal planes forwhich a ratio in area is determined are the (110) plane, and inaddition, the (012) plane, the (104) plane, the (113) plane, the (116)plane, the (300) plane, the (214) plane, and the (006) plane. From theresult shown in FIG. 6, it has been found that the α-Al₂O₃ layer ofsample no. 8 had a lower portion occupied in area at a ratio of 50% ormore by crystal grains having the (110) plane with a normal theretohaving a direction within ±15° with respect to the direction of thenormal to the second interface.

<<Cutting Test>>

(Cutting evaluation (1): Interrupted processing test) Using the cuttingtools of the samples (Samples 1 to 12 and Samples a to f) manufacturedas described above, a cutting time elapsing before a rake face had anα-Al₂O₃ layer peeled was measured. A result thereof is shown in Table 2.A cutting tool allowing a longer cutting time can be evaluated as acutting tool having larger peeling resistance.

Test conditions for interrupted processingWorkpiece: FCD700 grooved materialCutting speed: 150 m/minFeed rate: 0.25 mm/rev

Cutting Depth: 2 mm

Cutting oil: wet type

(Cutting Evaluation (2): Continuous Processing Test)

The cutting tools of the samples (Samples 1 to 12 and Samples a to f)manufactured as described above were used under the following cuttingconditions to cut a workpiece for 10 minutes and thereafter had theirflank faces measured for an amount worn on average. A result thereof isshown in Table 2. A cutting tool worn in a smaller amount on average canbe evaluated as a cutting tool having larger wear resistance.

Test Conditions for Continuous Processing

Workpiece: SCr440H round barCutting speed: 250 m/minFeed rate: 0.25 mm/rev

Cutting Depth: 2 mm

Cutting oil: wet type

TABLE 2 cutting cutting evaluation (2) EBSD color map analysis ofα-Al₂O₃ layer evaluation (1) worn amount (006) area ratio in (110) arearatio in (214) area ratio in cutting time on average samples upperportion (%) lower portion (%) lower portion (%) (sec) (mm) 1 80 85 2 4800.06 2 86 62 18 450 0.07 3 82 72 7 480 0.08 4 70 81 0 360 0.1 5 80 88 5390 0.1 6 88 72 10 270 0.13 7 75 63 21 330 0.09 8 82 52 8 360 0.11 9 5571 14 330 0.1 10 81 84 3 360 0.09 11 81 78 4 270 0.12 12 62 61 11 2400.14 a 32 25 58 30 0.39 b 51 11 62 60 0.29 c 42 15 68 60 0.27 d 20 3 7530 0.48 e 38 32 51 60 0.27 f 31 55 64 30 0.31

As can be seen in Table 2, the cutting tools of samples 1-12 (thecutting tools of the examples) provided a good result, that is, acutting time of 240 seconds or more in interrupted processing. Incontrast, the cutting tools of samples a to f (the cutting tools of thecomparative examples) provided a cutting time of 60 seconds or less ininterrupted processing. From the above results, it has been found thatthe cutting tools of the examples had larger peeling resistance thanthose of the comparative examples.

As can be seen in Table 2, the cutting tools of samples 1-12 (thecutting tools of the examples) provided a good result, that is, they hadtheir flank faces worn in an amount of 0.14 mm or less on average incontinuous processing. In contrast, the cutting tools of samples a to f(the cutting tools of the comparative examples) had their flank facesworn in an amount of 0.27 mm or more on average in continuousprocessing. From the above results, it has been found that the cuttingtools of the examples had excellent wear resistance.

Thus while embodiments and examples of the present invention have beendescribed, it is also initially planned to combine configurations of theembodiments and examples, as appropriate.

It should be understood that the embodiments and examples disclosedherein have been described for the purpose of illustration only and in anon-restrictive manner in any respect. The scope of the presentinvention is defined by the terms of the claims, rather than theembodiments and examples described above, and is intended to include anymodifications within the meaning and scope equivalent to the terms ofthe claims.

REFERENCE SIGNS LIST

-   -   1 rake face, 2 flank face, 3 cutting edge ridge portion, 10        substrate, 20 α-Al₂O₃ layer, 20 a first interface, 20 b second        interface, 20A lower portion, 20B upper portion, 21 inner layer,        22 intermediate layer, 23 outermost layer, 30 CVD apparatus, 31        substrate setting jig, 32 reaction chamber, 33 temperature        controller, 34 gas introduction port, 35 gas introduction pipe,        36 jetting hole, 40 coating, 50 cutting tool, A imaginary plane        A, B imaginary plane B, C imaginary plane C, D imaginary plane D

1. A cutting tool comprising a substrate and a coating that coats thesubstrate, the coating including an α-alumina layer provided on thesubstrate, the α-alumina layer including crystal grains of α-alumina,the α-alumina layer including a lower portion and an upper portion, thelower portion being a region sandwiched between an imaginary plane A andan imaginary plane B, the imaginary plane A being an imaginary planewhich passes through a point 0.2 μm away in a direction of thicknessfrom a first interface located on a side of the substrate and isparallel to the first interface, the imaginary plane B being animaginary plane which passes through a point 1.3 μm further away fromthe imaginary plane A in the direction of thickness and is parallel tothe first interface, the upper portion being a region sandwiched betweenan imaginary plane C and an imaginary plane D, the imaginary plane Cbeing an imaginary plane which passes through a point 0.5 μm away in thedirection of thickness from a second interface opposite to the side ofthe substrate and is parallel to the second interface, the imaginaryplane D being an imaginary plane which passes through a point 1 μmfurther away from the imaginary plane C in the direction of thicknessand is parallel to the second interface, the first interface beingparallel to the second interface, when a cross section of the α-aluminalayer obtained when cut along a plane including a normal to the secondinterface is subjected to an electron backscattering diffraction imageanalysis using a field emission scanning microscope to determine acrystal orientation of each of the crystal grains of α-alumina and acolor map is created based thereon, then, in the color map, the upperportion being occupied in area at a ratio of 50% or more by crystalgrains of α-alumina having a (006) plane with a normal thereto having adirection within ±15° with respect to a direction of the normal to thesecond interface, the lower portion being occupied in area at a ratio of50% or more by crystal grains of α-alumina having a (110) plane with anormal thereto having a direction within ±15° with respect to thedirection of the normal to the second interface, the α-alumina layerhaving a thickness of 3 μm or more and 20 μm or less.
 2. The cuttingtool according to claim 1, wherein the coating further includes an innerlayer provided between the substrate and the α-alumina layer, and theinner layer includes TiCN.
 3. The cutting tool according to claim 2,wherein the coating further includes an intermediate layer providedbetween the inner layer and the α-alumina layer, the intermediate layerincludes a compound composed of: elemental titanium; and at least oneelement selected from the group consisting of carbon, nitrogen, boronand oxygen, and the intermediate layer is different in composition fromthe inner layer.
 4. The cutting tool according to claim 1, wherein thecoating further includes an outermost layer provided on the α-aluminalayer, and the outermost layer includes a compound composed of: anelemental titanium; and one element selected from the group consistingof carbon, nitrogen, and boron.
 5. The cutting tool according to claim1, wherein the coating has a thickness of 3 μm or more and 30 μm orless.
 6. The cutting tool according to claim 1, wherein in the colormap, the upper portion is occupied in area at a ratio of 50% or more and90% or less by crystal grains of α-alumina having a (006) plane with anormal thereto having a direction within ±15° with respect to adirection of the normal to the second interface.
 7. The cutting toolaccording to claim 1, wherein in the color map, the lower portion isoccupied in area at a ratio of 50% or more and 90% or less by crystalgrains of α-alumina having a (110) plane with a normal thereto having adirection within ±15° with respect to a direction of the normal to thesecond interface.
 8. The cutting tool according to claim 2, wherein theinner layer has a thickness of 3 μm or more and 20 μm or less.
 9. Thecutting tool according to claim 3, wherein the intermediate layerincludes TiCNO, TiBN, or TiB₂.
 10. The cutting tool according to claim3, wherein the intermediate layer has a thickness of 0.3 μm or more and2.5 μm or less.
 11. The cutting tool according to claim 4, wherein theoutermost layer includes TiC, TiN, or TiB₂.
 12. The cutting toolaccording to claim 4, wherein the outermost layer has a thickness of 0.1μm or more and 1 μm or less.